1. Field of the Invention
The present invention generally relates to the application of metallic thin films for integrated circuit chip wiring and, more particularly, to a method of laser planarization of the metallic films requiting lower laser fluences.
2. Description of the Prior Art
Pulsed laser planarization of metallic thin films for on-chip wiring, including filling of high aspect ratio vias, is currently being developed. The technique relies on a high power, very short laser pulse to rapidly melt an optically absorbing metallic film. The molten metal is characterized by high surface tension and low viscosity which allows it to fill deep vias and planarize its surface.
The pulsed laser planarization technique suffers from the fact that low resistivity metals of interest, such as aluminum and alloys of aluminum and copper, are characterized by a high reflectivity. Thus, higher laser fluences are required to melt or nearly melt metallic conductors such as aluminum and alloys of aluminum and copper. The use of high laser fluences is inefficient and has been found to result in a higher degree of ablation which leads to discontinuous metallic lines and partially filled vias. The process window for achieving good results is narrow, making this process difficult for use in future manufacturing. Also, higher laser fluences result in higher temperatures to the surrounding dielectric media. This is of concern for organic interlevel dielectrics such as polyimides which rapidly degrade above 500.degree. C.
One effort to solve this problem has been to deposit an antireflection film, such as titanium, on aluminum lines prior to laser melting. The problem with this approach is that heating and melting the coated aluminum metallization by the laser beam causes the antireflection coating to intermix with the aluminum, resulting in contaminated high resistivity metallurgy.